Typically, each battery powered device requires its own charger and power source, which is usually an AC power outlet. This becomes unwieldy when many devices need charging.
Approaches are being developed that use over the air power transmission between a transmitter and the device to be charged. These generally fall into two categories. One is based on the coupling of plane wave radiation (also called far-field radiation) between a transmit antenna and receive antenna on the device to be charged which collects the radiated power and rectifies it for charging the battery. Antennas are generally of resonant length in order to improve the coupling efficiency. This approach suffers from the fact that the power coupling falls off quickly with distance between the antennas. So charging over reasonable distances (e.g., >1-2 m) becomes difficult. Additionally, since the system radiates plane waves, unintentional radiation can interfere with other systems if not properly controlled through filtering.
Other approaches are based on inductive coupling between a transmit antenna embedded, for example, in a “charging” mat or surface and a receive antenna plus rectifying circuit embedded in the host device to be charged. This approach has the disadvantage that the spacing between transmit and receive antennas must be very close (e.g. mms). Though this approach does have the capability to simultaneously charge multiple devices in the same area, this area is typically small, hence the user must locate the devices to a specific area.
A variety of low power devices, such as Bluetooth headsets and hearings aids, are on the market today. In a magnetically resonant wireless charging application, it is desired to transfer as much power as possible to a device being charged. For high power capacity devices, like a cellular telephone for example, a high power transfer capability is crucial so that the high power device can be charged in a short period of time. Low power capacity devices however, generally cannot accept high levels of power transfer without significant risk of permanent damage. There is a need for systems, methods, and devices related to charging both high and low power devices while reducing the risk of damage to the low power devices.